A conventional digital information transmission system contains a data source, a transmitter, a transmission medium, and a receiver. Illustratively, in a digital television system, the data source is a digitized audio-video signal, the transmitter contains a plurality of application encoders (e.g., a video signal encoder, an audio signal encoder, and a system control information encoder), a transport encoder for packetizing and multiplexing the encoded signals and an M-ary quadrature amplitude modulation (QAM) modulator. The transmission medium is typically a cable network or wireless path.
The receiver in a digital television system contains a demodulator for demodulating the QAM signal, a transport decoder for depacketizing and demultiplexing the encoded signals, a plurality of application decoders, and a presentation device for displaying the information from the data source to a user, e.g., the presentation device can be a conventional television. The demodulator produces a serial baseband digital signal (a bit stream containing packetized and multiplexed digital information). As is well-known in the art, the demodulator accomplishes carrier recovery, signal equalization, packet synchronization and the like, to generate a useful baseband digital signal. The baseband signal must be further processed by a transport decoder to extract from the baseband signal the video, audio and timing information within the data packets.
In a digital information transmission system employing bandedge timing recovery, an imbalance in the amplitudes of the upper and lower bandedge signal strength causes “stress” or jitter in the timing recovery circuitry. To produce jitter-free timing signals, such timing recovery circuitry rely on constant or near constant signal strength at both bandedges. If the incoming signal becomes attenuated, in some cases a 10 db difference between upper and lower bandedge signal strength can occur for broadband signals, timing signals are no longer produced in a jitter-free manner and the demodulator may be “thrown out of sync” with the rest of the system resulting in a degraded or non-existent baseband signal.
For example, in wireless communication systems, as the carrier frequency increases, the impact of multipath attenuation becomes more pronounced. Traditional equalizers in these types of systems can easily compensate for multipath attenuation by employing some standard form of closed end cancellation to eliminate the reflected signal that causes the incident signal attenuation. However, for broadband signals, a conventional equalizer does not compensate for different attenuation at each bandedge, e.g., an imbalanced bandedge signal strength. Consequently, the timing loop becomes “stressed” when the imbalanced signals are received. This results in jitter or unevenly spaced intervals of the timing signals. Properly spaced timing signals are critical to the optimal operation of the receiver.
Therefore, a need exists in the art for a method and apparatus that can autobalance the amplitudes of the bandedges of the incoming signal to reduce timing signal jitter caused by an imbalance in amplitudes of the upper and low bandedge signals.